The present invention relates to processing of food products, and more particularly to processing systems and methods for killing microorganisms in fluid food products or foodstuffs, which systems and methods extend the shelf life of such food products or foodstuffs. Even more particularly, the present invention relates to the prevention of electrode fouling in such systems and methods.
Substantial technical effort has been directed to the preservation of perishable fluid food products such as milk products, natural fruit juices and liquid egg products which may normally contain a wide variety of micro-organisms, and which are excellent culture media for microorganisms.
Practical preservation methods which have found significant commercial application predominantly utilize heat treatment such as pasteurization to inactivate or reduce the microorganism population. For example, milk products are conventionally pasteurized at a minimum temperature of at least about 72.degree. C. for 15 seconds (or equivalent time/temperature relationship) to destroy pathogenic bacteria and most of the nonpathogenic organisms, with degradative enzyme systems also being partially or totally inactivated. However, products processed in this manner are still generally nonsterile and have limited shelf life, even at refrigeration temperature. The shelf life of liquid foodstuffs may be substantially extended by higher heat treatment processes such as "ultra high pasteurization", or "ultra high temperature" ("UHT"). Such treatments vary in times and temperatures for different industries and food products. A typical treatment would be to maintain the product at a temperature of 140.degree. C. for four seconds. These processes are used in conjunction with aseptic packaging to achieve complete destruction of all bacteria and spores within the food product. However, such heat treatment typically adversely affects the flavor of the food product, at least partially denatures its protein content or otherwise adversely affects desired properties of the fluid food product. Other approaches to liquid food preservation, which also have certain disadvantages, include the use of chemical additives or ionizing radiation.
The bactericidal effects of electric currents have also been investigated since the end of the 19th century, with various efforts having been made to utilize electrical currents for treating food products. Such efforts are described in U.S. Pat. Nos. 1,900,509, 2,428,328, 2,428,329 and 4,457,221 and German Patents 1,946,267 and 2,907,887, all of which are incorporated herein by reference. The lethal effects of low-frequency alternating current with low electric field strength have been largely attributed to the formation of electrolytic chemical products from the application of current through direct contact electrodes, as well as ohmic heating produced by current flow through an electrically resistive medium. As described in U.S. Pat. No. 3,594,115, incorporated herein by reference, lethal effects of high voltage arc discharges have also been attributed to electrohydraulic shock waves. However, the electrolytic chemical products generated by low frequency, low electric field methods may be undesirable in fluid foodstuffs. Moreover, the utilization of explosive arc discharges to produce microbiologically lethal shock waves has not found wide-spread application as it is not a very effective means for preserving edible liquid foodstuffs and, in addition produces undesirable chemical by-products in the foodstuffs.
More recently, separately from the art of food preservation, the effect of strong electric fields on microorganisms in nonnutrient media has been studied as a mechanism for reversibly or irreversibly increasing the permeability of the cell membrane of microorganisms and individual cells. See, e.g., Sale, et al., "Effects of High Electric Fields on Microorganisms III. Lysis of Erythrocytes and Protoplasts", Biochmica et Biophysica Acta, 163, pp. 37-43, inter alia, (1968); Hulsheger, et al., "Killing of Bacteria with Electric Pulses of High Field Strength", Radiat. Environ Biophys, 20, pp. 53-65, inter alia, (1981); Hulsheger, et al., "Lethal Effects of High-Voltage Pulses on E. coli K12", Radiat. Environ. Biophys. 18, pp. 281-288, inter alia, (1980); Zimmermann, et al., "Effects of External Electrical Fields on Cell Membranes", Bioelectrochemistry and Bioenergetics, 3, pp. 58-63, inter alia, (1976); Zimmermann, et al., "Electric Field-Induced Cell-to-Cell Fusion", J. Membrane Biol., 67, pp. 165-182, inter alia, (1982); Hulsheger, et al., "Electric Field Effects on Bacteria and Yeast Cells", Radiat. Environ. Biophys; 22, pp. 149-162, inter alia, (1983); U. Zimmermann, et al., "The Development of Drug Carrier Systems: Electrical Field Induced Effects in Cell Membranes", Biochemistry and Bioenergetics, 7, pp. 553-574, inter alia, (1980); Jacob, et al., "Microbiological Implications of Electric Field Effects II. Inactivation of Yeast Cells and Repair of Their Cell Envelope", Zeitschrift fur Allgemeine Mikrobiologic, 21, 3, pp. 225-233, inter alia, (1981); Kinositas, Jr., "Formation and Resealing of Pores of Controlled Sizes in Human Erythrocyte Membrane", Nature, 268, 4, pp. 438-440, inter alia, (August, 1977); and Neamann, et al., "Gene Transfer into Mouse Lyoma Cells by Electroporation in High Electric Fields", IRI Press Limited, Oxford, England, pp. 841-845.
The application of high electric fields to reversibly increase the permeability of cells has been used to carry out cell fusion of living cells and to introduce normally excluded components into living cells. Electric fields in nonnutrient media can also have a direct irreversible lethal effect upon microorganisms with the rate of kill dependent upon the field strength above a critical field level and the duration of the applied high electric field.
These studies postulate the cell membrane as the site of a critical effect, of reversible or irreversible loss of membrane function as the semipermeable barrier between the cell and its environment. An external field of short duration is assumed to induce an imposed transmembrane potential above a critical electric field value, which may produce a dramatic increase of membrane permeability. Because an increase in cell permeability prevents the counteracting of differences in osmality of the cell content and surrounding media, exchange or loss of cell contents, cell lysis and irreversible destruction may occur as secondary mechanisms in nonnutrient media which limit the ability of cells to repair themselves, and which adversely affect permeable cells through osmotic pressure differences between the medium and the interior of the cell.
A pulsed field treatment apparatus, which uses very high electrical field pulses of very short duration, is shown in U.S. Pat. No. 5,048,404 issued to Bushnell et al. (the '404 Patent), incorporated herein by reference. Generally, in accordance with the '404 Patent, methods and apparatus are provided for preserving fluid foodstuffs (or pumpable foodstuffs), which are normally excellent bacteriological growth media, by applying very high electrical field pulses of very short duration through all of the foodstuff.
By "pumpable foodstuff" is meant an edible, food product having a viscosity or extrusion capacity such that the food product may be forced to flow through a treatment zone. The products include extrudable products, such as doughs or meat emulsions, fluid products such as beverages, fluid dairy products, gravies, sauces and soups, and food-particulate containing food slurries such as stews, and food-particulate containing soups, and cooked or uncooked vegetable or grain slurries.
By "bacteriological growth medium" is meant that upon storage at a temperature in the range of 0.degree. C. to about 30.degree. C., the fluid foodstuff, with its indigenous microbiological population or when seeded with test organisms, will demonstrate an increase in biological content or activity as a function of time as detectable by direct microscopic counts, colony forming units on appropriate secondary media, metabolic end product analyses, biological dry or wet weight or other qualitative or quantitative analytical methodology for monitoring increase in biological activity or content. For example, under such conditions the microbiological population of a pumpable foodstuff which is a bacteriological growth medium may at least double over a time period of two days.
The compositions of typical fluid food products which are biological growth media, derived from "Nutritive Value of American Foods in Common Units", Agriculture Handbook No. 456 of the U.S. Department of Agriculture (1975), are as follows:
______________________________________ FLUID FOODSTUFFS Fluid Carbo- Food Water Protein Fat hydrate Na K Product Wt % Wt % Wt % Wt % Wt % Wt % ______________________________________ Whole 87.4 3.48 3.48 4.91 .05 .144 Milk (3.5% fat) Yogurt** 89.0 3.40 1.68 5.22 .050 .142 Raw 88.3 .685 .20 10.0 .0008 .2 Orange Juice Grape 82.9 .001 tr. .166 .0019 .115 Juice Raw 91.0 .41 .20 8.0 .0008 .14 Lemon Juice Raw 90.0 .48 .08 9.18 .0008 .16 Grape- fruit Juice Apple 87.8 .08 tr. 11.9 .0008 .10 Juice Raw 73.7 12.88 11.50 .90 .12 .13 Whole Eggs Fresh 87.6 10.88 .02 .79 .15 .14 Egg Whites Split Pea 70.7 6.99 2.60 16.99 .77 .22 Soup* Tomato 81.0 1.60 2.10 12.69 .79 .187 Soup* Tomato 68.6 2.0 .588 25.4 1.04 .362 Catsup Veg- 91.9 2.08 .898 3.9 .427 .066 etable beef soup ______________________________________ *condensed commercial **from partially skimmed milk
Electrical fields may be applied by means of treatment cells of high field stability design which are described in detail by Bushnell et al. Basically, the foodstuff is electrically interposed between a first electrode, and a second electrode. The electrical field is generated between the first and second electrodes such that the electrical field passes through the foodstuff, thereby subjecting any microorganisms therein to the electrical field. Generally, the second electrode consists of a grounded electrode, and a relatively higher or lower voltage potential is applied to the first electrode. Various embodiments of such methods and apparatus use electric field processing to both preserve and heat for the combined benefit of electric field treatment at slightly elevated temperature. The use of de-gassing methods and apparatus to facilitate the use of high electric fields is another aspect of the '404 Patent discussed in more detail therein.
In the '404 Patent all of the pumpable fluid foodstuff is subjected to at least one very high field and current density electric pulse, and at least a portion of the fluid foodstuff is subjected to a plurality of very high voltage electric pulses in a high stability electric pulse treatment zone. The pumpable food product is subjected to such very high voltage short duration pulses by a variety of processing techniques. In one such processing method, the liquid foodstuff is introduced into a treatment zone, or cell, between two electrodes which have a configuration adapted to produce a substantially uniform electric field thereinbetween without dielectric tracking or other breakdown. Very high electric field pulses are applied to the electrodes to subject the liquid foodstuff to the multiple pulse treatment by pulsed field apparatus such as lumped transmission line circuits, Blumlein transmission circuits and/or capacitive discharge circuits. Alternatively, Bushnell et al., use field reversal techniques in capacitive discharge systems or pulse forming networks to increase the effective potential across the cell. For example, by applying a short electric field pulse of 20,000 volts per centimeter across a treatment cell for a short period of time (e.g., 2 microseconds) of one polarity, followed by abrupt reversal of the applied potential within a short time period (e.g., 2 microseconds), the '404 Patent achieves an effective field approaching 40 kilovolts per centimeter across the cell.
If the liquid foodstuff is continuously introduced into the treatment zone to which very high electric field pulses are periodically applied, and fluid foodstuff is concomitantly withdrawn from the treatment zone, the rate of passage of the liquid foodstuff through the treatment zone is coordinated with the pulse treatment rate so that all of the pumpable foodstuff is subjected to at least one electric field pulse within the treatment zone. The liquid foodstuff may be subjected to treatment in a sequential plurality of such treatment zones, or cells, as is described in more detail in the '404 Patent.
Problematically, in processing some food products, such as milk or rich protein solutions, using the apparatus and method of the '404 Patent or the like, a film of material can collect, or agglomerate, on the first and/or second electrode. This film of materials can consist of proteins and/or other materials (referred to herein as a fouling agent or polluting agent) that are present in the milk, or other protein rich material. The formation of the film, or fouling of the electrode(s), is believed to be due to the electrophoretic concentration of charged molecules within a boundary layer of food product that is adjacent to the treatment electrode. It has been noted that, for example, when the food product consists of raw milk, the fouling occurs only on the anode (i.e., the electrode to which electrons flow); the cathode (i.e., the electrode from which electrons flow) remains relatively free of any film buildup or agglomeration. Unfortunately, this agglomeration of the fouling agent on the electrode(s) during extended processing periods can cause electrical breakdown in the cell, fouling or contamination of the system, and in some cases can even cause the flow of fluid food product to stop. For some products, significant fouling of the electrode (or electrodes) can occur after only a few minutes of system operating time. For other products the time before which the fouling of the electrode (or electrodes) becomes significant can be a few hours or longer.
One attempt to solve a similar problem--electrolysis--is shown in U.S. Pat. No. 4,695,472, issued to Dunn and Pearlman. In accordance with the teachings of the '472 Patent, the suggestion is made that the first and second electrodes can be constructed so as to prevent direct electrolysis of the fluid foodstuff upon application of a pulsed electric field thereto. That is, such electrodes may comprise an electrically conductive electrolysis electrode, an ion permeable membrane and an intermediate electrolyte, such that ionic electrical connection is made with the fluid foodstuff through the ion permeable membrane rather than by direct contact with the electrically conductive electrode. Problematically, however, such electrolysis electrodes do not address the problem of electrophoresis, and they require the use of costly and cumbersome additional components in the pulsed field treatment apparatus.
Therefore, improvements are needed in the preservation of such high energy electrodes.